The present invention relates to a circuit breaker that serves as a breaker or a switching device for protecting a low-voltage indoor electric line or equipment from over-current. The circuit breaker has a function of opening and closing an electric line manually and tripping the electric line upon detection of the over-current.
A circuit breaker of this type has been disclosed in, for example, Japanese Patent Publication (Kokai) No. 2000-231869. A configuration of the circuit breaker will be explained with reference to FIG. 6. The circuit breaker shown in FIG. 6 is comprised of a molded case including an intermediate housing 1, a lower housing 2 connected to a lower part of the intermediate housing 1, and an upper cover (not shown) fitted to an upper part of the intermediate housing 1. Both the intermediate housing 1 and the lower housing 2 are formed in a rectangular box shape with a bottom. The lower housing 2 is connected to the lower part of the intermediate housing 1 via an engagement part (not shown) such that the lower housing 2 covers the lower part of the intermediate housing 1.
A left side in FIG. 6 is a power supply side, and a right side in FIG. 6 is a load side. Power supply side fixed contacts 3 and load side fixed contacts 4 are disposed at the bottom of the intermediate housing 1 such that they face each other in the longitudinal direction.
A power supply side terminal 5 is formed in a bent shape integrated with the power supply side fixed contact 3, which is fitted into the intermediate housing 1 from the left side in FIG. 6. The load side fixed contact 4 is fitted into a bimetal holder 6 made of an insulating material, and the bimetal holder 6 is fitted into the intermediate housing 1 from the right side in FIG. 6. In FIG. 6, reference numeral 7 denotes a time delay tripping mechanism having a bimetal 8 and a heater 9 wound around the bimetal 8. The bimetal 8 is supported in an upright position in the bimetal holder 6 via a bimetal support 10 made of a conductor. One end of the heater 9 is joined to an upper end of the bimetal 8, and the other end thereof is joined to the load side fixed contact 4.
Further, reference numeral 11 denotes an instantaneous tripping mechanism comprised of a plunger 14 capable of sliding up and down in a coil 13 disposed at an inside of a yoke 12. The instantaneous tripping mechanism is supported in an upright position in the intermediate housing 1 via the yoke 12. One end of the coil 13 is joined to the bimetal support 10, and the other end thereof is joined to the load side terminal 16 held in the bimetal holder 6 via a relay conductor 15.
A movable contact 17, which bridges the power supply side fixed contact 3 and the load side fixed contact 4, is housed in the lower housing 2. In the state shown in FIG. 6, the movable contact 17 is pressed against the power supply side fixed contact 3 and the load side fixed contact 4 to close an electric line by a contact spring 18 comprised of a compression spring inserted between the movable contact 17 and the bottom of the lower housing 2. The movable contact 17 is held in a movable contact holder 19 made of an insulating material, and the movable contact holder 19 is guided into the lower housing 2 to be slidable up and down. A fixed contact and a movable contact are joined at a contact portion between the fixed contact 3 and the movable contact 17, and a contact portion between the fixed contact 4 and the movable contact 17, respectively.
Extinguish chambers 20 are respectively provided at front and rear of the movable contact 17. Arc gas emission holes 21 each formed of several small holes, through which arc gas generated upon breaking of current flows out, are formed in front and rear walls of the lower housing 2, which are opposed to the respective extinguish chambers 20. Tongue-shaped projections 2a are formed in an upright position in respective upper parts of the front and rear walls of the lower housing 2. The power supply side fixed contact 3 and the bimetal holder 6 holding the load side fixed contact 4 are pressed by the projections 2a in the longitudinal direction so that they do not fall out.
A switching mechanism 23 is disposed in the intermediate housing 1. The switching mechanism 23 is comprised of a switching lever 26 that rotates around a shaft 25 in response to ON/OFF actions of a switching handle 24. When the switching handle 24 is in the ON state as shown in FIG. 6, the switching lever 26 is held in the state as shown in FIG. 6, and a main spring (not shown) comprised of a torsion spring attached to the shaft 25 holds elastic energy. When the switching handle 24 is brought into the OFF state from the ON state, the switching lever 26 rotates clockwise to push down the movable contact 17 via the movable contact holder 19. As a result, the movable contact 17 opens the electric line between the fixed contacts 3 and 4.
In the ON state as shown in FIG. 6, the current flows in a conductive path from the power supply side terminal 5 to a load side terminal 16 via the power supply side fixed contact 3, movable contact 17, load side fixed contact 4, heater 9, bimetal 8, bimetal support 10, coil 13, and relay conductor 15 in this order. When the load current flowing in this conductive path is overloaded, the heater 9 heats the bimetal 8 to deform and release the locked switching mechanism 23 via a shifter 27 upon the lapse of time according to a value of the current. As a result, the switching lever 26 is driven to rotate clockwise by the energy applied by the above-mentioned main spring, so that the movable contact 17 is opened to cut the current (tripping action).
When the large current such as short-circuit current flows in the above-mentioned conductive path, an electromagnetic repulsive force between the fixed contacts 3 and 4 and the movable contact 17 surpasses the force of the contact spring 18, so that the movable contact 17 is instantaneously driven in a direction to be released. At the same time, the plunger 14 is pulled by a magnetic field generated by the coil 13, and causes a push rod 28 to project downward to hold the movable contact 17 in the open position. Further, the plunger 14 releases the locked switching mechanism 23 via a trip board (not shown). As a result, just as in the case where the bimetal 8 is deformed, the switching lever 25 pushes down the movable contact holder 19, so that the movable contact 17 is held in the open position even after the electromagnetic force disappears.
In such a tripping action by the flow of the large current, high-voltage arc gas is generated inside the lower housing 2 due to an arc generated between the fixed contact and the movable contact.
The above-mentioned arc gas passes through the extinguish chambers 20 to reach the walls of the lower housing 2 on the power supply side and the load side, and then is emitted through the gas emission holes 21 as indicated by arrows in FIG. 7. When the large current is cut or broken, the lower housing 2 is deformed due to an increase in the internal pressure thereof caused by the arc gas. For this reason, the lower case 2 covering the intermediate case 1 may be disengaged from the intermediate case 1.
It is therefore an object of the present invention to prevent the lower housing 2 from detaching from the intermediate housing 1 due to an increase in the internal pressure when the large current is broken.
Further objects and advantages of the invention will be apparent from the following description of the invention.